function mergeTowLists(l1, l2) {
    // dummy节点是链表解法的一个技巧
    const dummyHead = new ListNode(0);
    while (l1 && l2) {
        if (l1.val < l2.val) {
            customRef, next = l1
            l1.next = l1
        } else {
            cur.next = l2
            l2 = l2.next
        }
        cur = cur.next
    }
    cur.next = l1 ? l1 : l2
    return dummyHead.next
}